A SAW (Surface Acoustic Wave) device having a comb-like electrode for exciting a surface acoustic wave on a piezoelectric substrate, for example, is used as a component for frequency adjustment and selection of a mobile phone.
For this surface acoustic wave device, a piezoelectric material such as lithium tantalate, LiTaO3 (hereinafter also referred to as “LT”), and lithium niobate, LiNbO3 (hereinafter also referred to as “LN”), is used to make it, because piezoelectric materials meet the requirements of smallness in size, small insertion loss, and ability to stop passage of unnecessary waves. In particular, the communication standards of cellular phones of the fourth generation often have a narrow differential in frequency band between transmission and reception, and a wide bandwidth. Furthermore, since the properties of the material of a surface acoustic wave device undergo changes with temperature, causing a shift in the frequency selection range, there occur problems in the functions of the filter and duplexer. Therefore, a material for a surface acoustic wave device, which has small tendency to undergo fluctuation in characteristics with respect to temperature change, and has a wide band, or is stable against a narrow temperature fluctuation, is desired.
Regarding the material for the surface acoustic wave device, for example, IP Document 1 teaches that a stoichiometric composition LT composed of copper used as an electrode material and mainly obtained by a gas phase method is preferable because the breakdown mode which is destroyed at the moment when high power is input to the IDT electrode is difficult to occur. IP Document 2 has a detailed description on the stoichiometry composition LT obtained by the gas phase method; and IP Document 3 describes a method of forming a waveguide for annealing a waveguide formed in a ferroelectric crystal of lithium tantalate or lithium niobate; and IP Document 4 describes a piezoelectric substrate for a surface acoustic wave device obtained by subjecting a lithium tantalate or lithium niobate single crystal substrate to Li diffusion treatment. IP Document 5 and Non-IP Document 1 also report that when LT in which the LT composition is uniformly transformed to Li-rich in the thickness direction by the gas phase equilibrium method is used to make the surface acoustic wave element, its frequency temperature characteristic is improved, which is preferable.
However, the inventors of the present invention have examined the methods described in these publications, and as a result, it has been found that these methods do not necessarily provide favorable results. In particular, according to the method described in IP Document 5, since the wafer is processed over a long period of time of 60 hours at a high temperature of about 1300° C. in the vapor phase, the manufacturing temperature has to be high, the consequent warpage of the wafer is large, and cracks occur at high rate, whereby the productivity becomes poor, and there is also a problem that the product becomes too expensive as a material for a surface acoustic wave device. Moreover, the degrees of variation in characteristics become large on account of the facts that the vapor pressure of Li2O is low and the modification degree of the sample to be modified varies depending on the distance from the Li source, and hence a considerable improvement is required for industrialization, and this problem has not been solved yet.
Also, with respect to a rotated Y-cut lithium tantalate single crystal substrate for SAW device, in order to apply this to a mass production process, it is necessary to adjust the sound speed of the shear horizontal type (SH) wave used in the device to be within a desired range. Therefore, Non-IP Document 2 describes means to adjust the acoustic (sound) velocity of the SH wave to be within a certain range, such as a method of adjusting the acoustic velocity of the SH wave by restricting the Curie temperature of the lithium tantalate single crystal substrate within a certain range to thereby adjusting the acoustic velocity of the SH wave, making use of the fact that the said Curie temperature and the acoustic velocity of the SH wave correlate with each other; a method of adjusting the acoustic velocity of the SH wave by restricting the lattice constant of the lithium tantalate single crystal substrate within a certain range thereby adjusting the acoustic velocity of the SH wave, making use of the fact that the said lattice constant and the acoustic velocity of SH wave correlate with each other; and a method of adjusting the acoustic velocity of SH wave by restricting the acoustic velocity of the shear vertical type acoustic wave of the lithium tantalate single crystal substrate within a certain range to thereby adjusting the acoustic velocity of the SH wave, making use of the fact that the said acoustic velocity of the shear vertical type acoustic wave and the acoustic velocity correlate with each other.
However, in the case of a rotated Y-cut lithium tantalate single crystal substrate having such a Li concentration profile in which the Li concentration differs between the substrate surface area and the inside of the substrate on account of the fact that Li was diffused into the LiTaO3 substrate from the surface to the inside thereof, since a Li concentration distribution occurs in the depth direction, the Curie temperature and the lattice constant also fluctuate in the depth direction due to the said Li concentration distribution, there is consequently a problem that it is difficult to use the Curie temperature and the lattice constant as the indices for adjusting the sound speed of the shear horizontal type (SH) wave to be within a desired range. Furthermore, the relationship between the lithium tantalate single crystal substrate having the Li concentration profile of varying values between the substrate surface and the inside of the substrate and the acoustic velocity of the shear vertical type (SV) acoustic wave is not well known yet.